1. Field of the Invention
The invention relates to a transport device for edge-perforated recording support materials comprising two casings, disposed at the perforated edges, for drivable endless pin belts with, respectively, a guide flap for the recording support materials.
2. Brief Description of the Background of the Invention Including Prior Art
Such transport devices designated as push or pull tractors are employed in printers of all types, in particular in matrix printers, for a very accurate paper advance performable in very precise steps. The transport devices are disposed in mirror-image position on a driven shaft and on a guide rod running parallel to the driven shaft and they can be set to the respective width of the paper provided with the perforated edges. In order to achieve an accurate paper advance, the same advance force is always required, which is transferred via the pins of the pin belts. Such transport devices include only few parts and, in general, do not need any maintenance.
Such a paper tractor is known from U.S. Pat. No. 4,199,091. The paper tractor is provided with a pin belt, having a frame of injection-molded material, where a plate pair is provided where, in a side-by-side disposition, one plate exhibits a hub, which hub protrudes at the side, whereby the second plate is disposed oppositely and is provided with inner and outer guide ribs for the pin belt. In addition, a mechanism for the detachable attachment of the two plates is known, where openings are provided through the inner guide rib, which engage in blocking latches, which blocking latches in turn are provided on the opposite plate. In order to generate such blocking latches by injection molding of plastic material, it is required to guide through the respective plate a larger mold bore in order to be able to form the injection molding tool correspondingly. However, as has been shown in practice, such blocking latches are associated with substantial disadvantages. In case of a disengagement of the two plates, the blocking latches have to be bent very far in order to release the locking engagement. In this process, the plastic is frequently excessively stressed and overloaded by inexperienced personnel and operators such that the blocking latches break off.